Research on repetitive genetic elements of humans and other mammals was continued. Alu sequences are derived from small transposed elements endogenous to the human genome. Nearly one million dispersed copies constitute 5% of the genetic material. Alu mobility has caused genetic variability and multiple heritable disorders in humans. The regulated expression of Alu and its mouse homologue B1 into RNA was studied. Germline and fetal-specific expression suggests their involvement in normal development. Examination of mechanisms of B1 and Alu RNA expression continued to yield novel results and insight into their complex regulation and propensity for transposition. 1. Advances in transcriptional regulation were made. A transcription termination/reinitiation factor for RNA polymerase III was characterized. Methods to isolate native transcription complexes by affinity chromotography and to improve the study of transcriptional regulation were developed. 2. Protein factors that interact with these repetitive DNA elements to regulate their expression, as well as proteins that modulate the metabolism of their RNA products were characterized. 3. An Alu RNA binding protein was purified to apparent homogeneity and a partial amino acid sequence was determined. From this information a cDNA was isolated and characterized. 4. Sequences that encode several small RNAs were mapped to single human chromosomes.